GCD with static functions of a struct

Question

how\'d you apply thread safe functionality to static functions of a struct

class SingleSome {

    struct Static {
        private static var instance: Singl         


        

Answer 1

Use a semaphore, dispatch_sync isn't appropriate because you need a synchronous return value from getInstance:

class SingleSome {

    struct Static {
        private static var instance: SingleSome?
        private static let lock = dispatch_semaphore_create(1)

        //need barrier sync
        static func getInstance(block: () -> SingleSome) -> SingleSome {
            dispatch_semaphore_wait(lock, DISPATCH_TIME_FOREVER)
            var value = instance
            if value == nil {
                instance = block()
                value = instance
            }
            dispatch_semaphore_signal(lock)
            return value!
        }

        static func remove() { //need barrier sync
            dispatch_semaphore_wait(lock, DISPATCH_TIME_FOREVER)
            instance = nil
            dispatch_semaphore_signal(lock)
        }
    }

}

Also note that as written this is subject to deadlocks if block results in either remove or getInstance being called as dispatch_semaphore_t is not thread recursive.